One type of imaging system employs photoconductive materials to absorb incident radiation representative of an image of an object. Suitable photoconductive materials will absorb the radiation and produce electron-hole pairs (charge carriers) which may be separated from each other by an electric field applied across the photoconductor, creating a latent image at the surface of the photoconductor (which is typically a thin planar layer). A narrow beam of scanning radiation substantially completes discharge of the photoconductor, by creating motion of a second set of charge carriers. The distribution of these second charge carriers in the plane of the photoconductor is affected by the distribution of the first charge carriers, i.e., by the latent image. The motion of the second charge carriers is detected and digitized in an appropriate circuit, and thus the latent image is captured in digital form.
In one specific embodiment, the photoconductor is part of a multilayer structure comprising two electrodes, between which are the photoconductive layer and an insulating layer. A high voltage power supply maintains electric fields in the structure during exposures to the incident radiation and the scanning radiation (although not necessarily the same field strength is present during each exposure). An example of this type of system is taught in U.S. Pat. No. 4,176,275 (Korn et al.). Application of the electric field across the photoconductive layer can be assisted by establishing a prior (reverse) field across the insulating layer, as taught in U.S. Pat. No. 4,539,591 (Zermeno et al.).
A second and closely related approach, known as the air-gap photo-induced discharge (PID) method, employs air as the insulating layer, and requires that a uniform separation be maintained between the two electrodes, typically by high-precision mechanical or piezoelectric devices. A corona charges the surface of the photoconductor prior to exposure to radiation, producing an electric field in the material. Thus, the incident radiation partially discharges the surface to produce a latent image, and the read-out signal is induced by the charge motion under the influence of the residual electric field in response to the scanning radiation. Such a system is described in Rowlands et al., Med. Phys. 18(3), May/Jun 1991 at 421-431.
Various methods for scanning the latent image exist. For example, the method of U.S. Pat. No. 4,961,209 (Rowlands et al.) employs a transparent sensor electrode positioned over the photoconductive layer, and a pulsed laser which scans the photoconductive layer through the transparent sensor electrode. By moving the photoconductive layer and the transparent sensor relative to each other, so that the direction of relative motion is transverse to the direction in which the laser scans, a pixel-by-pixel discharge of the charge carriers is created.
Practical applications of these systems have encountered several problems.
First, fabrication of the imaging stack (i.e., the electrodes, insulator, photoconductive material, etc.) requires applying layers of material to each other, typically by constructing two sub-stacks, and then applying them to each other. These procedures can introduce non-uniformities into the thicknesses of the imaging stack.
Second, reflection and scattering of incident radiation can occur at the interfaces between layers, reducing image quality. This problem, and the attempted solutions to it, are compounded by the non-uniformities in thicknesses.
Third, discharge breakdown of the insulative material is possible, especially in the air-gap PID approach, leading to avalanche currents in the system.
Fourth, as the areal size of the imaging stack increases, a requirement of practical applications such as chest x-ray imaging, the capacitance created by the electrode plates increases, reducing the effectiveness of the system. One approach to this last problem is that of U.S. Pat. No. 4,857,723 (Modisette). This approach avoids, rather than solves, the capacitance issue, by employing many small detectors ganged together.